Application of a current pulse to a thermal ink jet printer, as described for example in U.S. patent application Ser. No. 292,841, filed on Aug. 14, 1981 by Vaught et al, causes an ink droplet to be ejected by heating a resistor located within an ink supply. This resistive heating causes a bubble to form in the ink and the resultant pressure increase forces the desired ink droplet from the printhead. Thermal ink jet printer life time is dependent upon resistor life time and a majority of resistor failures result from cavitation damage which occurs during bubble collapse. In order to make multiple printhead, e.g., page width, arrays economically feasible, it is important that cavitation damage be minimized and that thermal jet ink jet printer life times exceed at least one billion droplet ejections.
In accordance with the illustrated preferred embodiment of the present invention, a thermal ink jet printer is shown in which cavitation damage is minimized and an extended life time is achieved. A printhead resistor is utilized which has a central conductive portion surrounded by a region of resistive material. Thus, a cold spot occurs in the center of the resistor when the current pulse is applied and a toroidal bubble is grown in the ink. During collapse, the bubble fragments into numerous smaller bubbles and the shock of the bubble collapse is randomly distributed across the resistor surface instead of being concentrated in a small central area.